The pyramidal tract (PT) transmits information from extensive regions in motor, premotor and sensory cerebral cortex to multiple structures in the brain stem and spinal cord. It participates in motor functions and very likely in other, not yet elucidated functions, indicated by its extensive origins in nonmotor cortex and more massive projections to sensory than to motor nuclei. The purpose of this study is threefold: 1) to determine in the cerebral cortex of cats and monkeys the sites of origin of the total PT, 2) to establish the morphology of individual PT neurons, 3) to utilize this study for an evaluation of two versions of the HRP (horseradish peroxidase) technique, used for retrograde tracing of neural pathways. HRP will be applied in the medulla to all axons that constitute the pyramidal tract. After retrograde HRP transport and appropriate histochemistry, pyramidal tract neurons are visualized in situ, in serial brain sections. From these data, total extent of PT cortex and subregions within will be delimited and laminar location and percentage of PT cells in each subregion established. A neuron's laminar location determines to a large extent its functional position within the local neural network. In each subregion, individual neurons will be traced to derive, with the aid of computer analysis, some size and shape, number and orientation of primary dendrites. An independent morphological profile for each subregion is important for microelectrode studies (usually focused on small subregions) to determine relations between neuron structure and function. Comparison of number and size of PT neurons with number and size of PT axons (to which HRP was applied) will reveal what fraction of the PT-population is labelled by a given HRP method and whether the labelled sample is biased on the basis of soma size.